PART 4: BEHAVIORAL PLASTICITY #24: SPATIAL NAVIGATION IN RATS I spatial learning cells that code for space synaptic plasticity in the hippocampus experiments that are knockouts summary

PART 4: BEHAVIORAL PLASTICITY #24: SPATIAL NAVIGATION IN RATS I spatial learning cells that code for space synaptic plasticity in the hippocampus experiments that are knockouts summary

SPATIAL LEARNING rats... order Rodentia family Muridae subfamily Murinae... all common mice & rats cosmopolitan arboreal, terrestrial & even aquatic nocturnal (generally)

two most common species black house rat... Rattus rattus SPATIAL LEARNING two most common species black house rat... Rattus rattus brown house rat... Rattus norvegicus really good motor and navigation skills 

SPATIAL LEARNING navigation by cues... two types, both used  path integration using memory of familiar environment missing sensory cues (e.g., visual) use internally generated ideothetic cues... vestibular proprioceptive optic flow (deprived in this e.g.)  external landmarks learning the environment direct sensory input (e.g. visual observation)

SPATIAL LEARNING – MAZE LEARNING navigation by learning in a novel environment how do animals learn this task? complex problem rat used to study behavioral strategies neural mechanisms maze learning...

SPATIAL LEARNING – MAZE LEARNING maze learning among earliest experiments with rats Tolman (1930s) cognitive maps for solving spatial problems internal representations for current problems extend to novel problems also, first guy to study inheritance of behavior

SPATIAL LEARNING – MAZE LEARNING radial arm maze reward at end of each arm optimal foraging how many visits until all 8 food items retrieved? training few days... 7/8 ~ 3 weeks... ~8/8

SPATIAL LEARNING – MAZE LEARNING 2 types of memory in this experiment  working memory used within single trials which arms have yet to be visited? experiments to test different possible strategies visible landmarks rules... e.g., clockwise marking visited arms rats establish map of maze over time

SPATIAL LEARNING – MAZE LEARNING 2 types of memory in this experiment  reference memory used between trials memory of food items in specific arms remember positions between trials

SPATIAL LEARNING – MAZE LEARNING Morris water maze - spatial learning must find hidden platform relative to visible cues in room training... trials test reference memory rat encodes spatial relationships QUADRANTS

SPATIAL LEARNING – MAZE LEARNING Morris water maze - spatial learning must find hidden platform relative to visible cues in room training... trials test reference memory rat encodes spatial relationships QUADRANTS

SPATIAL LEARNING – MAZE LEARNING Morris water maze - cued learning associates visible platform with reward (escape) QUADRANTS

SPATIAL LEARNING – MAZE LEARNING circular platform maze ~ terrestrial version of Morris water maze rats prefer to hide rather than be exposed 1 hole  access to hidden chamber test reference memory ~ visible cues in room

SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS rat brain

SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS hippocamus

SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS

SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS hippocampal formation = hippoc. + associated bits hippocampus... “sea horse” (Greek) major connections: entorhinal cortex & fornix

SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS regular longitudinal internal circuitry (~ MB)  pyramidal cells in single layer similar array in dentate gyrus, associated structure  granule cells in single layer    

SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS 3 main subdivisions: CA1, CA2 (not shown), CA3 main pathways;  = trisynaptic circuit  perforant path: entorhinal cortex  dentate gyrus  mossy fibers: granule  CA3 cells  Schaffer collaterals: CA3  CA1 cells  commissural path: through fimbria  CA1 highly oversimplified additional circuits feed-back & -forward ~ rest of brain

SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS limbic system – humans

SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS hippocampal lesions – humans, other mammals not all memories lost memories of facts or events (explicit, declarative, episodic) impaired or lost post-lesion memory impaired only pre-lesion memory intact procedural, implicit memories ok important for acquisition & memory of types of new information spatial information in particular LIMBIC SYSTEM

SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS effects of hippocampal lesions – rats, radial arm maze cued learning (informed that arm visited) ok spatial impaired* perseveration *

SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS effects of hippocampal lesions – rats, water maze spatial impairment  longer circuitous routes swimming impaired or just enjoy a good swim? 3-part experiment  spatial task  impaired ability to find platform  cued task  can swim, would rather not  spatial task  still impaired... no memory!

CODING SPACE – HIPPOCAMPAL PLACE CELLS extracellular recordings from hippocampus in freely moving rats firing of neurons ~ spatial position in chamber, e.g. B many neurons ~ spatial position  place field maps “complex spike cells”... CA1 & CA3 (as it turned out)

CODING SPACE – HIPPOCAMPAL PLACE CELLS cues governing a place cell’s spatial preference free moving rats, chamber, “permanent” electrodes, camera from above recording position ~ space place field maps differences in preference, size & shape of fields, some cells (D) do not behave like place cells (strictly)

CODING SPACE – HIPPOCAMPAL PLACE CELLS pattern not innate... acquired, then ~ stable  weeks cells can have multiple preferences preferences shaped by contexts (e.g., rewards) gradual drift in preference ~ changing context

CODING SPACE – HIPPOCAMPAL PLACE CELLS cells fire ~ cues move cues, cell fires ~ preferred cue new position remove cues, cell fires ~ memory of preferred cue position... “deduced reckoning” or ideothetic cues what are these internal ideothetic cues? movement related... vestibular & visual test in striped cylinder; walls & floor rotation separately showed that animals own movement  place cell firing

CODING SPACE – HIPPOCAMPAL PLACE CELLS what happens in the dark (without visual cues)? train animal in light, turn out lights place cell activity remains ~ 30 min after lights out ~ memory train animal in light, remove, return to dark chamber cell established a new place field train animal in light, remove, return to dark chamber, turn on the lights retains remapped field determined in dark preference ~ real time or memory of cue preferences

CODING SPACE – HIPPOCAMPAL PLACE CELLS another cool place cell experiment... cell readings made during all parts of experiment rats trains in 4-arm maze ~ visual cues could find food relative to position of cues in test next, trained in 4-arm maze ~ visual cues cues removed before test, had to remember place when tested correctly, cells fired as before in arm when tested incorrectly, cells fired incorrectly suggests that if cell firing  (processing... memory... )  behavior 

CODING SPACE – HIPPOCAMPAL PLACE CELLS place cells encode more than simple space T-maze, trained (fruit loops) to alternate L & R turns subset of place cells showed interesting pattern e.g., activity (sector 3) anticipating right turns only suggests hippocampal network represents episodic memories, cells are small segments of an episode link of cells with overlapping episodes  memories

CODING SPACE – HIPPOCAMPAL PLACE CELLS spatial dreaming large # space cells only ~ 15% active in any 1 environ. some silent in one environ., active in others time- & labor-intensive to get larger picture device to measure 150 cells at once population or ensemble code code predicts rat behavior in maze many environments & codes overlapping, not interfering used to study plasticity...

CODING SPACE – HIPPOCAMPAL PLACE CELLS spatial dreaming plasticity strengthening of code  learning accompanied by reduced inhibitory activity does code relate to consolidated (permanent) memory trained rats in spatial task measured code during training sleeping before training sleeping after training dreaming replay of events  memory consolidation

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